Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 22 2023 15:58:24 FOUS30 KWBC 221558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1057 AM EST Fri Dec 22 2023 Day 1 Valid 16Z Fri Dec 22 2023 - 12Z Sat Dec 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN PORTIONS OF CALIFORNIA AND ARIZONA... ....16Z Update... No major changes to the outlined risk areas from previous forecast issuance as the current synoptic evolution of our closed low remains on track. Newest probabilistic guidance from the 12Z HREF signals a pretty decent convective pattern later this afternoon across the Colorado River Basin starting in Imperial and eastern Riverside counties in CA, moving east into Yuma/La Paz in AZ with greater than 90% potential for neighborhood reflectivity of at least 40 dbz with 50 dbz coming in at 60-70% over the aforementioned area. This correlates well with the 2-year ARI exceedance probability of >90% over much of Yuma/La Paz counties extending northeast into the I-10 corridor in AZ, and the 10-year ARI running up to 60% as of the latest guidance. This is likely due to the increased ascent pattern of the low traversing overhead within low-level moisture advection on the lead side of the low. PWAT indices on the latest mesoanalysis indicate a tongue of 1+" values extending from western Sonora up over to the CA Baja and points north. This places the Colorado River Basin as the focal for deeper moisture presence, thus better chance for higher rain rates once convection ensues. Steep lapse rates off the KSGX sounding this morning peg a picture of the expected instability pattern that will transpire for points east. Neighborhood 1"/hr rain rates have bumped to near 70% probability across the above area, so expectation is for locally heavy rainfall with totals between 1-2" and a max of up to 2.5" as a possibility today which signals flood concerns rising. Further east, the steady plume of low-mid level moisture will continue to advect northeast into south-central AZ with moderate to locally heavy rainfall anticipated for several hrs before the pattern shifts to the east as the low migrates towards the Four Corners area. Tongue of instability can be found within the corridor of higher IVT's lifting northward within the axis of difluence on the lead side of the broad circulation. This will maintain the possibility of localized flooding due to the onslaught of rain with weak convection embedded in the precip field. Best chance for flooding within that zone will be within the urbanized areas and low-water crossings over the low desert within Pima county up to the north of the Tuscon metro. The SLGT risk will suffice given the potential.=20 Kleebauer ....Previous Discussion... The deep, closed low centered just offshore southern California/northern Baja early Friday morning will begin to pick up speed as it moves eastward toward central Arizona by early Saturday morning, opening up as it does so. This will put much of southern California, far southern Nevada, and Arizona in a favored region of deep-layer forcing/lift while anomalous S-SSW low-level flow positions a plume of higher moisture initially into southern California then across Arizona during the afternoon/evening hours. Moisture anomalies with this system are expected to remain well above normal, 3 to 4 standard deviations above normal for this time of year for both PW and 850-700 mb moisture flux. PWs in fact are forecast to reach or exceed 1" at times, especially across southwest to south-central Arizona. The GFS and ECMWF IVT forecasts from CW3E show an impressive surge approaching 500+ kg/m/s into central Arizona. During the afternoon, some limited instability may develop across the Desert regions of CA and eastward into southern Arizona, which may allow for some localized higher rain rates approaching 0.50-1.0"/hr at times per the 00Z CAM guidance. Overall, a broad area of 0.75-1.5" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 2.5" possible in any thunderstorms. The latest (00Z 12/22) HREF neighborhood probabilities of 24hr QPF exceeding the 2 year ARI are aoa 50 percent for the bulk of the Slight area, with probs of exceeding the 10 year ARI up to 40-60% along the southern CA-AZ border. Hurley Day 2 Valid 12Z Sat Dec 23 2023 - 12Z Sun Dec 24 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE RED RIVER VALLEY SOUTH TO THE MIDDLE AND UPPER TEXAS COAST... Full-latitude upper trough traverses the Rockies and moves into the High Plains late on day 2 (00-12Z Sun). During this time (late Day 2...Sat night), deep-layer ascent and south-to-north moisture flux will strengthen fairly rapidly along the Red River Valley south through North TX and much of the TX Triangle to the Middle-Upper TX Coast. This as the forcing becomes enhanced from both the subtropical upper level jet (left-exit region) and S-N oriented upper jet streak across the Central Plains (right-entrance region). PWs and 850 mb moisture flux anomalies in fact become 2 to 3 standard deviations above normal Sat night per the 00Z GEFS and SREF. The signal per the latest 00Z guidance remains bullish on the prospects of multiple areas of convection developing within the theta-E advection pattern extending from the Texas Gulf Coast into central Oklahoma ahead of the mean trough, though there continues to be quite a bit of areal spread (including the 2 CAMs that go out through the entire D2 period -- NAM CONUS-Nest and FV3). Rich low-level theta-e/moisture pooled from the western Gulf advecting north within the confines of a 30-50kt LLJ will allow for a focal point of convective development and maintenance as the broad ascent pattern shifts eastward through the southern plains. 00Z ECMWF is most bullish in terms of the degree of deep-layer instability and QPF along the Mid-Upper TX Coast, with a footprint of 3-5+ inches noted through 12Z Sun. Maintained the MRGL for the time being, but would not be surprised if at least a portion of the outlook area is bumped up to a Slight should we see better agreement on the placement of the heaviest QPF footprint. Hurley/Kleebauer Day 3 Valid 12Z Sun Dec 24 2023 - 12Z Mon Dec 25 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN OZARKS AND ARKANSAS RIVER VALLEY SOUTH THROUGH THE ARKLATEX AND CENTRAL GULF COAST REGION... Aforementioned mid/upper trough is expected to close off at 500 mb by 12Z Christmas Day across the Plains. Strong southern stream enhancement (130-150 kt upper jet streak moving across South TX) will boost the deep-layer ascent and south-north ageos low-level moisture/theta-e transport into eastern OK-TX as well as much of AR-LA and the MS-AL Gulf Coast regions. PWs >1.5" begin to creep into the Upper TX Coast region early in the period, then across southern LA and coastal MS later Sun-Sun night. The anomalous low-level transport, aligned low-mid level flow with an anomalous LLJ (favoring increased upwind propagation/backbuilding and thus risk for cell training), and at least modest deep-layer instability (at least 500-1000 J/Kg) will lead to a favorable thermodynamic and kinematic profile for heavy rainfall, especially given the active, robust southern stream which is indicative of the current strong warm-phase ENSO pattern. As such, for the most part we have maintained both the Slight and Marginal Risk areas with minimal changes from yesterday's Day 4 ERO. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9v2-B3Z7ARl-SLZgKzni3tJxYCQMLmI2JoCD_6qByOwU= oqa98V_eIJMqcM0py_XFOzCVM6sCqyjMYF6BTzKgIAMG1aw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9v2-B3Z7ARl-SLZgKzni3tJxYCQMLmI2JoCD_6qByOwU= oqa98V_eIJMqcM0py_XFOzCVM6sCqyjMYF6BTzKgmM8Fn3U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9v2-B3Z7ARl-SLZgKzni3tJxYCQMLmI2JoCD_6qByOwU= oqa98V_eIJMqcM0py_XFOzCVM6sCqyjMYF6BTzKgMyp2XbY$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .